you can leave it connected as long as you like....with the truck off, both batteries will equilize (power transfer will stop when the dead battery reaches the same voltage as the truck battery) with the truck running, the trucks voltage regulator will keep both batteries from overcharging....easy breezy.
Also if you bring a few extra 12V batteries (never a bad idea in mex) and a inverter (12V dc to 120V ac), you can connect the spare batteries in parallel (pos. to pos, neg. to neg.) and charge the whole bank with jumpers (this may take an hr+ to fully chare) then at night you dont have to run the generator (or even bring it depending on who power hungry your camp will be). We have even pre-wired, secured the batts and enclosed it all in a simple wooden enclosure so eveything stays put and doesnt take up much room in the back of the truck cause you can stack on it....and if you use some cheap, used batteries, it also makes a very nice gift to some hombre suerte when you're ready to leave.
